1. Field of the Invention
The present invention relates generally to improvements in planar-magnetic speakers. More particularly, the invention relates to magnetic circuit configurations for single-ended and double-ended devices.
2. Background
Two general fields of loudspeaker design comprise (i) dynamic, cone devices and (ii) electrostatic thin-film devices. A third, heretofore less-exploited area of acoustic reproduction technology is that of thin-film, fringe-field, planar-magnetic speakers.
This third area represents a bridging technology between these two previously recognized areas of speaker design; combining a magnetic motor of the dynamic/cone transducer with the film-type diaphragm of the electrostatic device. However, it has not heretofore produced conventional planar-magnetic speakers, which, as a group, have achieved a significant level of market acceptance over the past 40-plus years of evolution. Indeed, planar-magnetic speakers currently comprise well under 1% of the total loudspeaker market. It is a field of acoustic technology which has remained exploratory, and embodied in only a limited number of relatively high-priced commercial products over this time period.
As with market acceptance of any speaker, competitive issues are usually controlling. In addition to providing performance and quality, a truly competitive speaker must be reasonable in price, practical in size and weight, and must be robust and reliable. Assuming that two different speakers provide comparable audio output, the deciding factors in realizing a successful market penetration will usually include price, convenience, and aesthetic appearance. Price is obviously primarily a function of market factors such as cost of materials and cost of assembly, perceived desirability from the consumer's standpoint (as distinguished from actual quality and performance), demand for the product, and supply of the product. Convenience embodies considerations of adaptation of the product for how the speaker will be used, such as mobility, weight, size, and suitability for a customer-desired location of use. Finally, the aesthetic aspects of the speaker will be of consumer interest; including considerations of appeal of the design, compatibility with decor, size, and simply its appearance in relation to the surroundings at the point of sale and at the location of use. If planar-magnetic speakers can be advanced so as to compare favorably with conventional electrodynamic and electrostatic speakers in these areas of consideration, further market penetration can be possible, as reasonable consumers should adopt the product that provides the most value (bearing in mind the aforesaid factors, for example) for the purchase price paid.
A discussion of the relative successes and failures of conventional planar-magnetic speakers, and design goals and desired traits of operation will be set forth. It is interesting to note that the category of fringe-field, planar-magnetic speakers has evolved around two basic categories: single-ended; and, symmetrical double-ended designs, the latter sometimes being called “push-pull.”
A conventional double-ended, or push-pull, device is illustrated in FIG. 1. This structure is characterized by two magnetic arrays 10 and 11 supported by perforate substrates 14, 24 positioned on opposite sides of a flexible diaphragm 12, which includes a conductive coil 13. The film is tensioned into a planar configuration. An audio signal is supplied to the coil 13, and a variable voltage and current thereby provided in the coil gives rise to a variable magnetic field, which interacts with the fixed magnetic field set up by and between the magnet arrays 10 and 11. The diaphragm is displaced in accordance with the audio signal, thereby generating a desired acoustic output. An example representing this art area is found in U.S. Pat. No. 4,156,801 issued to Whelan.
Because of a doubled-up, front/back magnet layout of the prior art push-pull magnetic structures, double-ended systems have been generally regarded as more efficient, but also as more complex to build. Also, they have certain performance limitations stemming from the formation of cavity resonances arising from passage of sound waves through cavities or channels 16 formed by the spacing of the magnets of the magnet arrays 10,11 and the holes 15 in the substrates 14, 24. This can cause resonant peaks and band-limiting attenuation at certain frequencies or frequency ranges.
Double-ended designs are also particularly sensitive to deformation from repulsive magnetic forces that tend to deform the devices outward. Outward bowing draws the edges of the diaphragm closer together, and alters the tension of the diaphragm. This can seriously degrade performance; and, over time, can render the speaker unusable.
As mentioned, another category of planar-magnetic speakers comprises single-ended devices. With reference to FIG. 2, a typical conventional single-ended speaker configuration, having a flexible diaphragm 17 with a number of conductive elements 18, is illustrates prior art design. The diaphragm is tensioned and supported by frame members (not shown) carried by a substrate 19 of the frame, which frame extends outward and upward in the figure beyond a single array of magnets 20 to position the diaphragm a gap or offset distance away from the faces (tops in the figure) of the magnets to accommodate vibration of the diaphragm. The magnet array provides a fixed magnetic field with respect to coil conductors 18 disposed on the diaphragm. It will be apparent that the single array of magnets (typically of ceramic or rubberized ferrite composition) provides a much-reduced energy field compared with previously-discussed push-pull devices, assuming comparable magnets are used. Previous single-ended devices of compact size have generally not been deemed acceptable for commercial applications.
Conventional single-ended devices have had to be quite large to work effectively; and even so, were less efficient than standard electrostatic and electro-dynamic cone-type loudspeaker designs mentioned above. Small, or even average-sized single-ended planar-magnetic devices (compared to standard sizes of conventional speakers) have not effectively participated in the loudspeaker market in the time since introduction of planar-magnetic speakers. Very large devices, generally greater than 300 square inches, have been available to the consumers in the speaker market; and these exhibit limited competitiveness. That is to say, they are on par with standard speakers in terms of acceptance, suitability for certain applications, cost, and performance. But again, prior single-ended planar-magnetic devices with such large diaphragm areas require correspondingly relatively large, expensive structures; and, such relatively large speakers can be cumbersome to place in some domestic environments. They have relatively low efficiencies as well, compared with conventional electrostatic and dynamic transducers, requiring more powerful, and hence more expensive, amplifiers to provide adequate signal strength to drive them.
At first impression, a single-ended device might appear to be simpler and cheaper to build than a double-ended design. The same amount of magnet material can be used by doubling the thickness of the magnets to correspond to the combined thickness of a double-ended array of magnets. Because magnets which are twice as thick are cheaper than twice as many magnets half as thick in a double-ended device, there should be significant savings in a single-ended configuration. Furthermore, the structural complexity is significantly less with regard to single-ended designs, further adding to expected cost savings.
However, doubling the depth of the magnets from that of most designs does not achieve the desired design goal of providing twice the magnetic energy in the gap between the diaphragm and the array of magnets using conventional ferrite magnets used in prior planar-magnetic devices. Accordingly, the expectation for lower cost and better performance in the single-ended device has not been realized. Some attempts to improve the design of single-ended planar-magnetic devices have involved the use of many, very closely spaced, magnets, to have high enough magnetic energy. Even then, however, the planar area must be very large, using even more magnets to generate enough sensitivity and acoustic output. For at least these reasons, prior attempts to develop a commercially acceptable single-ended planar-magnetic device have not achieved the desired lower-cost design goals. This is true even though the basic form of their structure would seem to be simpler than push-pull devices.
The architecture of the double-ended planar-magnetic loudspeaker is quite different from that of a single-ended design. For example, the magnetic circuits of the front and back magnetic structures interact, and require a different set of parameters, spacing, and relationships between the essential elements to be optimized, for best results. This double-ended magnetic relationship causes greater repulsion forces, making it more difficult to have a stable mechanical structure, but also gives a more focused field, which can make for better utilization of magnetic material. Very few of those interactive relationships are transferable in relation to design of single-ended transducers, which have their own unique set of optimal relationships between the essential elements involved.
As mentioned, prior planar-magnetic speakers, particularly prior art single-ended devices, have utilized rows of magnets placed closely, side by side. The magnets are oriented with alternating polarities facing the film diaphragm, which includes conductive wires or strips 18 substantially centered between the magnets. Such prior devices further illustrate that the magnet energy to be captured by the conductive strips is a shared magnetic field with lines of force arching between adjacent magnets. In such prior devices, the magnetic force is assumed to be at a maximum at a point halfway between two adjacent magnets of opposite polarity orientation and, correspondingly, centered placement of the conductive strips in the field at that location is typical. To achieve this maximized flux density at the position centered between the magnets, it has been shown that (i) not only does the total size of the system need to be increased; but, (ii) the magnet placement must be much closer together and more plentiful in a single-ended device than in a push-pull planar-magnetic transducer.
Further, in contrast with standard, dynamic cone-type speakers, thin film planar loudspeakers have a critical parameter that must be optimized for proper functionality. The parameter is film diaphragm tension. (See, for example, U.S. Pat. No. 4,803,733) Proper, consistent and long-term stable tensioning of the diaphragm in a planar device is very important to the performance of the loudspeaker. This has been a problematic area for thin-film planar devices for many years, and it is a problem in the design and manufacture of current thin-film devices. Even the most carefully adjusted device can meet short-term specification requirements, but can still have long-term problems with tension changes due to the dimensional instability of the diaphragm material and/or diaphragm mounting structure. Compounding this problem is force interaction within the magnet array structure. Due to close magnet spacing of single-ended magnetic structures, the magnetic forces generated by adjacent rows of magnets can interact and attract/repel each other to a greater or lesser degree, depending upon factors such as the inter-magnet spacing and polarity relationship of the magnets. This interaction, over time, can cause materials to deform; and can impose changes on the film tension. This can degrade the performance of the speakers over time. Electrostatic loudspeakers have critical diaphragm tension issues, but they do not have relatively large magnetic forces working to change the tension in the same way or to the same degree. Dynamic cone-type speakers have magnetics and strong related forces, but generally do not utilize tensioned diaphragms. Planar-magnetic speakers pose unique challenges with respect to long-term stability for diaphragm tensioning.
With conventional planar-magnetics an increase in magnetic energy derived by increasing the number, or the strength, or both, of the magnets in the magnetic structure further exacerbates the problem of magnetic forces interference with calibrated film tension. Per the foregoing, this is true particularly over time. These and other problems are known in the art. An example of a prior art single-sided planar-magnetic device is set forth in U.S. Pat. No. 3,919,499 to Winey.
Turning now more particularly to consideration of the magnets themselves, the selection of proper magnets for planar-magnetic speakers is an important consideration. High-energy neodymium magnets have been available for over ten years, and have been used in electrodynamic cone-type speakers. As will be appreciated, however, such speakers do not employ magnetic materials structures, and supporting structures to support the magnets; and, at the same time, maintain a tension on the diaphragm that can be influenced by deformation, which can, in turn, be caused by the magnets. Such relatively more high-energy neodymium magnets have not been effectively applied to single-ended planar-magnetic transducers over this past decade, although they have been widely available. This is true even though there has been a great need for an improved magnetic circuit to enhance speaker output and reduce size.
With current magnetic structure designs having very close side-to-side spacing, a perceived problem with high-energy magnets is that the attractive forces would appear to be too intense, to a point of not only potentially distorting the supporting structure and affecting diaphragm tension, but even affecting stability of existing magnet attachment means. For these and other reasons such high-strength magnets have not been used in commercial conventional planar-magnetic transducer design.
As mentioned, particularly with double-ended devices, cavity resonances and other distortion problems arise due to the narrow channels between magnets, radiating to the outside through holes in the support structure. Single-ended devices, particularly where the magnet spacing is close, and the cavities between the magnets is relatively deep and narrow, also have been subject to distortions, particularly at the high and low frequency portion of their performance envelope. At least in part, this is also due to the close spacing of the magnets in prior devices, with attendant band limiting attenuation and resonances arising from the geometry of the cavities and holes through the supporting structure.
Also important is the magnetic circuit configuration and its relationship to the diaphragm conductive regions. The maximization of the interaction between coil and magnetic structure is key to gaining better efficiency, and can improve response, particularly at lower frequencies. Also, thermal and dimensional stability of the diaphragm material is important to performance, particularly over a long time of product use. Likewise the incorporation of the coil in or on the diaphragm is important. If the coil conductors de-bond, develop an open circuit (for example by fatigue failure), speaker performance is compromised. With both single- and double-ended devices, other considerations apply, but these give some background as to the design challenges faced. Single-ended and double-ended devices both have drawbacks and advantages relative to each other and overall both have previously been perceived to have both advantages and disadvantages compared with conventional electrostatic and electrodynamic cone-type devices. However, both single- and double-ended planar-magnetic transducers have continued to lag behind conventional cone type and electrostatic speakers in maximizing the use of magnetic drive and finding commercial acceptance.
In summary, heretofore neither conventional double-ended or single-ended designs of planar-magnetic loudspeakers have reached a stage of development which enables them to be competitive with speakers of the first two types discussed above (dynamic and electrostatic), the latter previously having higher efficiencies and lower manufacturing costs. This lack of market success, due at least in part to the reasons set out above, has continued over a period of more than 40 years.